Dormant oil is a horticultural oil sprayed on plants during their dormant period. The oil naturally repels, discourages feeding, and suffocates problematic pests. Additionally, some fungal diseases like powdery mildew can be controlled.

The use of dormant oil is a safe and effective way to help control pests such as aphids, caterpillar eggs, mites, scale, thrips and whiteflies while less likely having a negative impact on beneficial insects.

Originally, dormant oil applications were sprayed on fruit trees as a safe alternative to reducing damaging pests because many pesticides could not be sprayed during the growing season for fear of eradicating beneficial insects; especially those that pollinate the fruit. So, dormant oils were the solution.

Now, with improved technology, there are pesticides that are safe to apply during the pollination period, but dormant oils are still an effective method to control problematic pests during the dormant season.

There are two types of dormant oils: vegetable-based and petroleum-based. Organic gardeners prefer vegetable-based dormant oil because residue and toxicity are minimized.

Dormant oils can be sprayed on fruit trees, shade trees and woody ornamental plants that have experienced pest problems during the growing season.

Although dormant oils evaporate quickly, use caution when applying to trees such as Maple, Redbud, Hickory, and Walnut. Phytotoxicity can occur. One example of phytotoxicity damage is leaf burn which occurs if the oil retains or attracts too much heat. Blackened branches or yellowing foliage is the result of this burn.

Take care when applying dormant oil on certain evergreens. In particular, apply as little as possible on a shady day and follow up with water to keep the foliage from becoming stressed. Additionally, plants under drought stress have an increased risk of injury and can experience leaf burn.

You are the best judge of when and if you should spray your plants. Plants should be sprayed if there were pest problems throughout the previous year. The best time to apply dormant oil is during the winter when buds are swollen but not yet open. To prevent damage, do not apply dormant oils during extreme hot or cold temperatures; if plants are wet; humidity is high; or if sulfur or pesticides containing sulfur have been applied to the plant in the past month.

Now that you know some of the basics, apply dormant oil as a layer of prevention and a source of protection for your plants. Always read the label before spraying.